John, I can also design an adapter for the MagnaKeyer, but I'll need a very clear top-down image to reasonably gauge clearances.
For now, have a look at the virtual reality 3D images of a Vibroplex Iambic adapter I just created. It's at the very bottom of my QRZ page, accessed through the link below:
www.w9ac.comTwo images are attached, one of the top and the other is of the bottom. Notice that the 3.5mm jack is on the right, adjacent to the dah binding post. While I could make it come out the center in the form of a slightly triangular PC board, it adds unnecessary depth to the overall paddle and can expose the jack to damage. Have a look at a similar layout I did for a HamKey paddle. Scroll up from the bottom to view it.
I oversized each binding post pad to 0.170-inch. Frankly, I am surprised that the distances of the outer binding posts to the center post isn't equal. That's quite a drilling variance. The final version may elongate the holes into ovals in order to accommodate the excessive drilling variance for other key owners.
Let me know if this design works for you.
Paul, W9AC